The production of semiconductor devices requires precise control of material properties. Often, the production of such devices involves controlled heating of semiconductor materials. For instance, during the production process, semiconductor wafers (and other devices) may be subjected to an ion implantation process or processes that dope the wafer with impurities. After the ions have been implanted, the crystal lattice structure of the wafer is annealed by heating the wafer to a high temperature. Other typical processes utilizing heat treatment include growth and deposition of film layers, crystallization, and phase change processes.
However, heat treatment, especially at higher temperatures, can have many undesirable side effects. For example, in an annealing process, the high temperatures may lead to an undesired diffusion of the dopant atoms, which will lead to unpredicted or unwanted semiconductor properties. While such diffusion may occur in the course of any heat treatment process, it may be especially troublesome when a pulse of energy is used to heat-treat a wafer.
If the pulse of energy is delivered to the wafer surface over a time scale that is short, relative to the time that is necessary for heat to diffuse through the thickness of the substrate, then the surface of the wafer will become substantially hotter than the opposite surface. For a typical silicon wafer that is ˜775 μm thick, that is at a temperature of ˜800° C., such surface heating is achieved when the energy is absorbed within a region that is less than 200 μm below the surface of the wafer, when the pulse is of less than ˜20 ms duration. For a large effect, the energy should be absorbed within less than 20 μm from the surface and the pulse typically should be less than 5 ms in duration.
Immediately after the energy from the pulse is absorbed, the heat diffuses through the thickness of the wafer, raising the average temperature of the wafer as a result. This temperature rise occurs over a timescale defined by the rate of heat diffusion through the thickness of the wafer, which is typically ˜50 ms for a wafer at 800° C. that is 775 μm thick, as is typical for 300 mm diameter wafers. Hence pulsed heating produces a rather rapid rise in the temperature of the whole of the wafer. The subsequent evolution of the wafer temperature depends on the nature of the heat loss from the surfaces of the wafer into the surroundings. For example, if the back of the wafer is held in thermal contact with a heatsink structure, heat may be conducted away to the heatsink through the gap between the wafer and the heatsink. Alternatively, if the wafer is only supported at a few locations, and there are no cooler surfaces nearby, then heat may be lost from the surfaces by convection or conduction into the gas ambient surrounding the wafer, and by the emission of thermal radiation.
The rate at which heat is lost from the surface into the bulk of the wafer is typically very high, because thermal conduction within a solid is a very rapid and efficient mechanism of heat transfer. In contrast, transfer of heat from the wafer surface almost inevitably involves a thermal contact resistance that impedes the conduction of heat out of the wafer surface into a surrounding medium, or the relatively inefficient heat transfer mechanisms of convection or radiation. As a result, the bulk of the wafer tends to heat up after the pulse, and the thermal exposure (sometimes referred to as thermal budget) of the delicate structures within the wafer may be increased to an undesirable extent. Such thermal exposure may have deleterious effects such as introducing excessive diffusion of dopant species that have been introduced into the wafer. As a result there is a need to identify improved ways of limiting the thermal exposure of the wafer.
It is often desirable to combine the pulsed heating of the wafer surface with a second from of heating, called background heating, that preheats the wafer prior to application of the energy pulse. Typically such heating is useful because it enables the use of a lower energy pulse to achieve any desired degree of heating of the side of the wafer that is exposed to the pulsed heating process. The magnitude of the temperature pulse associated with the pulsed heating is also reduced, which leads to reduced stress within the wafer, as well as reducing the magnitude of any non-uniformity of heating associated with non-uniformity in the delivery of the energy pulse to the wafer surface. An example of the latter arises when the wafer is coated with materials that vary in composition across the surface that is exposed to pulsed heating. When such non-uniform coatings are exposed to an energy pulse, the magnitude of the resulting temperature non-uniformity decreases as the magnitude of the energy pulse decreases. Furthermore the ability to preheat the wafer allows the design of more sophisticated heating cycles, including the possibility of ramping this background temperature up to a given value and then applying the surface heating energy pulse. After the pulse, the background heating can be decreased and the wafer can cool. Various approaches are described in U.S. Pat. No. 6,849,831 and U.S. Pat. No. 6,594,446.
The ability to cool the wafer rapidly after the pulse of surface heating is clearly important for reducing the thermal exposure. Methods for limiting thermal exposure and improving cooling are described in U.S. Pat. No. 6,594,446, U.S. Pat. No. 5,561,735, U.S. patent application Ser. No. 10/629,400 filed Jul. 28, 2003, U.S. patent application Ser. No. 10/706,367 filed Nov. 12, 2003, U.S. patent application Ser. No. 10/646,144 filed Aug. 22, 2003, and U.S. patent application Ser. No. 09/527,873 filed Mar. 17, 2000.